$(i)$ Under normal conditions, what is the rate of breathing per minute for an adult? (ii) Why does the rate of breathing increase significantly during vigorous exercise?

(N/A) $(i)$ The normal breathing rate for a healthy adult at rest is $12$ to $20$ breaths per minute.
(ii) During vigorous exercise, the muscles require more energy to perform physical work. This leads to an increased demand for oxygen for cellular respiration and a need to remove excess $CO_2$ produced. Consequently, the breathing rate increases to supply more oxygen to the blood and tissues.